Make "HeaderName" and "ReadmeName" be stored in simple entries "header" and "readme" respectively.

Side effect:
In the case that multiple "HeaderName"s or multiple "ReadmeName"s exist in one directory section.
Before this change, the first directive is effective.
After this change, the last directive is effective.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@729314 13f79535-47bb-0310-9956-ffa450edef68